The present invention relates to a fuse, and particularly to a fuse which is applicable to a fuse link for breaking a circuit wiring provided in vehicles when an overcurrent flows through the wiring.
Generally, a fuse link for vehicles is used as a component provided in a circuit wiring around a power source of a vehicle for protecting the circuit wiring from a large amount of current to which the so-called glass-tube-type fuse can not be applied in the light of capacity. Ordinarily, the fuse link is made of a conductive material whose sectional area is smaller than that of the wire constructing the circuit wiring, and is thus effective for dead short. Incidentally, JASO-D610 provides the structure of such a fuse link.
Because the fuse link of this type can be regarded as a fuse in a broad sense, we call them "fuse" collectively hereinafter.
As a conventional fuse which is typical of the one as described above, there can be mentioned a fuse disclosed in Japanese Patent Laid Open No. 315924-1989. The fuse of this type is constructed by fixing a linear metal chip having a low melting point on a meltable and conductive metal plate (element).
The matrix of the meltable and conductive metal is an alloy of copper (Cu) which is the same as adopted in the circuit wiring. Incidentally, the Cu alloy contains Fe, P and the like component in a slight amount.
On the other hand, the material of the low-melting-point metal chip is tin (Sn) whose melting point is lower than that of Cu. Incidentally, the purity of Sn of the material is 99.5% by weight or higher.
Next, the mechanism of breaking by such a fuse is considered.
First, the chip of low-melting-point metal (Sn) is melted by an overcurrent. Then, the meltable and conductive metal element is gradually eroded by diffusion of Cu into the melted metal chip or Sn. Finally, the metal element itself is completely cut by such Cu-Sn diffusion. As the result, the breaking by this fuse can be carried out at a temperature lower than the melting point of Cu.
However, in case of such a conventional fuse, since the chip mounted on the Cu-alloy element is made of Sn having a low melting point, the fuse is likely to blow in a relatively short operating cycle regardless of the necessity.
For example, in case of opening or closing power windows of a car-door, a current about twice the rated current or in a so-called medium current range flows frequently and repeatedly in a short time period (e.g., 10 seconds) as a motor-lock current. However, the fuse must not blow in such a medium current range. Nevertheless, the Sn chip melts enough by such a current, thereby to cause the Cu-Sn diffusion as mentioned above. Therefore, the conventional fuse also blows in a relatively short operating cycle even under such a condition.
Accordingly, the durability, especially in the medium current range, is now considered to be an important problem among the blowout properties of such a fuse.